Electric tools, such as electric drivers, for use in the screw fastening work have hitherto been used primarily in construction sites and the like for business purposes as they permit significant improvement in operative efficiency. In recent years, they are available at home improvement centers and the like, and have come to be generally used for home carpentry.
In home carpentry, the price of an electric tool per one operation is expensive because of a low frequency of the use thereof. Thus, electric tools have been required to provide some sort of added value. Moreover, since portable radios are often brought into working operation sites, there have been proposed configurations in which a radio is added to a charger for charging a battery of an electric tool (e.g. Japanese Laid-Open Patent Publication No. 2000-92726: corresponding U.S. Pat. No. 6,427,070 and U.S. Pat. No. 6,496,688). However, in operation sites used by operation workers, commercial power supplies are often not prepared. Further, it is often the case that a customer having ordered working operations bears electrical expenses thereof. For these reasons, in many cases, spare batteries are prepared in working operation sites and batteries are not charged. It is therefore necessary to add some sort of value not to a charger, but to an electric tool itself.
Since a person holds an electric tool with his hand while at work, however, it needs to-be light enough for portability. This has made it impossible to add an additional function which would cause a weight increase of an electric tool.